X-ray reflectivity (XR) and grazing incident-angle x-ray diffraction (CIXD) reveal that sphingomyelin, although forming homogeneous monomolecular films at air-water interfaces, does not develop long-range crystalline in-plane order at any in-plane density, most notably at high surface pressures where many other membrane lipids order as two-dimensional crystals. Studies were carried out on the monodisperse synthetic C18-sphingomyelin where both hydrocarbon chains are saturated and on natural sphingomyelin which contains a distribution of chain lengths, with saturated and unsaturated hydrocarbon chains. The surface pressure versus molecular area (#pi#-A) isotherm for the natural sphingomyelin is similar to that of the synthetic, but has a somewhat higher surface pressure at intermediate areas. The absence of in-plane crystalline order is attributed to competing interactions inthe head group region: hydrogen bonding inthe erythro region, electrostatic interactions among zwitterions of neighboring head groups, and van der Waals among acyl chains.
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